1. Field of the Invention
This invention relates to a process for producing dimethyl ether from methyl alcohol (methanol). In particular, this invention relates to a process for producing dimethyl ether by vapor-phase dehydration of methanol in the presence of a catalyst.
2. Description of the Related Art
Dimethyl ether (DME) has been used as a propellant for spraying, and has recently attract attention as an alternative fuel to LPG or diesel oil, which does not generate particulate matters. It can be produced by, for example, direct synthesis from a synthesis gas prepared by reforming hydrocarbon materials such as methane, or dehydration of methanol.
Among these, a process for producing DME by dehydration of methanol and a catalyst used therefor have been disclosed in the references described below.
JP-A-03-056433 (1991) has disclosed a process for producing DME using a porous γ-alumina catalyst having a surface area of 210 to 300 m2/g, preferably 230 to 290 m2/g, a volume of pores with radius smaller than 300 Å of 0.60 to 0.90 mL/g, preferably 0.62 to 0.85 mL/g and an average pore radius of 50 to 100 Å, preferably 50 to 85 Å. This process is accomplished based on a finding that porous γ-alumina having a surface area, a pore distribution and an average pore radius in the particular ranges exhibits long-term stability. According to the examples described in this publication, a reactant is considered to be gaseous (vapor phase) methanol.
JP-A-59-016845 (1984) has disclosed a process for producing DME using an alumina catalyst having a surface area of 180 to 220 m2/g, preferably 180 to 200 m2/g, a pore volume 0.58 to 0.75 mL/g, preferably 0.60 to 0.75 mL/g, an average pore radius of 500 to 1000 nm, preferably 600 to 750 nm, a mode pore radius of 650 to 1000 nm, preferably 750 to 900 nm and an iron oxide (III) content of 0.5 wt % or less, preferably 0.1 wt % or less. In this technique, a pore volume of the catalyst is increased to ensure improved selectivity at a higher conversion ratio. According to the examples described in this publication, a reactant is considered to be gaseous (vapor phase) methanol.
JP-A-01-160933 (1989) has disclosed a process for producing DME wherein 5 to 45 parts by weight of steam or water is added to 100 parts by weight of methyl alcohol during vapor-phase dehydration of methyl alcohol using a γ-alumina catalyst for producing dimethyl ether. In this technique, steam or water is added to methyl alcohol in advance to considerably reduce carbon deposition on the surface of the dehydration catalyst for maintaining higher activity of the catalyst for a long term. This publication has described that a catalyst used in the process for producing DME is a γ-alumina catalyst, preferably a pure γ-alumina catalyst having lower impurity contents, i.e., 0.3% or less of silica, 0.03% or less of iron oxides and 0.10% or less of sodium oxides and having a surface area of 150 to 300 mm2/g.
JP-A-02-085224 (1990) has disclosed a process for producing DME using an alumina catalyst containing at is least one metal oxide selected from Group 3A elements in the periodic table during vapor-phase dehydration of methyl alcohol for producing dimethyl ether. In this technique, an alumina catalyst containing at least one metal oxide selected from Group 3A elements in the periodic table can be used to achieve higher activity at a low temperature without carbon deposition for a long term and with formation of hydrocarbon byproducts being reduced. The publication has described that in terms of the catalyst used in the process for producing DME, a content of the metal oxide added is in the range of 0.005 to 80 wt %, preferably 0.5 to 20 wt % to the total weight of the catalyst; the pure alumina used is preferably γ-alumina having lower impurity contents, i.e., 0.3% or less of silica, 0.03% or less of iron oxides and 0.1% or less of sodium oxides and having a surface area of 100 to 700 m2/g.